From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-9.8 required=3.0 tests=BAYES_00,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS,INCLUDES_PATCH, MAILING_LIST_MULTI,SIGNED_OFF_BY,SPF_HELO_NONE,SPF_PASS,URIBL_BLOCKED autolearn=unavailable autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id D64D8C4742C for ; Mon, 2 Nov 2020 17:08:06 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 7125A22280 for ; Mon, 2 Nov 2020 17:08:06 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=pass (2048-bit key) header.d=linaro.org header.i=@linaro.org header.b="JXzV2p0v" Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1727617AbgKBRIF (ORCPT ); Mon, 2 Nov 2020 12:08:05 -0500 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:45542 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1727221AbgKBRIE (ORCPT ); Mon, 2 Nov 2020 12:08:04 -0500 Received: from mail-ot1-x341.google.com (mail-ot1-x341.google.com [IPv6:2607:f8b0:4864:20::341]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 9943AC061A04 for ; Mon, 2 Nov 2020 09:08:04 -0800 (PST) Received: by mail-ot1-x341.google.com with SMTP id 79so5805556otc.7 for ; Mon, 02 Nov 2020 09:08:04 -0800 (PST) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=linaro.org; s=google; h=date:from:to:cc:subject:message-id:references:mime-version :content-disposition:in-reply-to; bh=h835lHTQAu95wxA6PWFRRi1Mf/8MD2a5rJ3lZGCt1cc=; b=JXzV2p0vzQ8oWk1hcaXFG0FvtdrXkV2dRVgA/IbVQE/hgmv5n5pCVIvX3ESBoyvRGf tgtfY7MWJUbRHG067Bs69792lkKZW5h9t+tEeDcB6BRq9PRAy76YtZ1WQ93f5nn5qQkz h/9uGzvP827kIjRUVNnANCl7bQ4nnsPVFpKVQknecDyRFRRGcqSTOfEjWdipbFNiKj9K 6/v0/FxOSV95gWgZNzOpV+sb61Bq0be7/mu8AiAVV3H916cU3YOXRmnVEShMx06rfuXz B42vxCATidn5x/xPrejMPvHKUB+LnQe0ofCsZgj42RK/AQoodaBKYalUoN9NELtSuyav 4l6Q== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20161025; h=x-gm-message-state:date:from:to:cc:subject:message-id:references :mime-version:content-disposition:in-reply-to; bh=h835lHTQAu95wxA6PWFRRi1Mf/8MD2a5rJ3lZGCt1cc=; b=F9F4pktRj1WoNepuvcowOoc4DW6A77uDsVakoR/XtIhpVW2uhTVXPqkPGEj7RJkt8d kWXVsr4d/4oEGEu7NZhQHMuWxbsrXSI0kbLwNhA0+hv1C9tB9DxfJ5juVVEvgB0MB5Vm F5yY5iJ65anTA/RKzgqV0I/wxt5DzZUinmaIkyoS50lsAnVJdST65zuK1QbCOP3KivYc KPXb7RtZVDWZlev4KqaK5r+0nkFH9WQEWhAKX5/F0amh2/1gNy5Td4uQV0yKqdRYR3Df oxKAPlAW06N3iLMm+BmTtmzwsKy5jVlG93epIZqbLxkk2fPHYNMnN4cgMKbVAVMVTGdA TFdw== X-Gm-Message-State: AOAM530E252Esxfgb/68XJbcQvjd+C15olnOw7xblNyikb18UnH24EXt k/vJtp3pBFGutRrw1H7boEHI+A== X-Google-Smtp-Source: ABdhPJwPghsbeLdlR1n488K/KOrm0Vc5OL56hGZgaK6freCCeb7B/2DFK47J9LaHVmCOZ7tOA3hd9g== X-Received: by 2002:a05:6830:2018:: with SMTP id e24mr13147614otp.278.1604336883838; Mon, 02 Nov 2020 09:08:03 -0800 (PST) Received: from builder.lan (104-57-184-186.lightspeed.austtx.sbcglobal.net. [104.57.184.186]) by smtp.gmail.com with ESMTPSA id y22sm3560488ooa.2.2020.11.02.09.08.02 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Mon, 02 Nov 2020 09:08:03 -0800 (PST) Date: Mon, 2 Nov 2020 11:08:01 -0600 From: Bjorn Andersson To: Doug Anderson Cc: David Airlie , Daniel Vetter , Rob Herring , Andrzej Hajda , Neil Armstrong , Laurent Pinchart , Jonas Karlman , Jernej Skrabec , dri-devel , "open list:OPEN FIRMWARE AND FLATTENED DEVICE TREE BINDINGS" , LKML , linux-arm-msm Subject: Re: [PATCH 1/2] dt-bindings: drm/bridge: ti-sn65dsi86: Replace #pwm-cells Message-ID: <20201102170801.GI3151@builder.lan> References: <20200930223532.77755-1-bjorn.andersson@linaro.org> <20200930223532.77755-2-bjorn.andersson@linaro.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Fri 02 Oct 15:42 CDT 2020, Doug Anderson wrote: > Hi, > > On Wed, Sep 30, 2020 at 3:40 PM Bjorn Andersson > wrote: > > > > While the signal on GPIO4 to drive the backlight controller indeed is > > pulse width modulated its purpose is specifically to control the > > brightness of a backlight. > > I'm a bit on the fence about this. I guess you're doing this because > it avoids some -EPROBE_DEFER cycles in Linux? It does seem to have a > few downsides, though. > No, the reason for exposing a backlight is that while the thing certainly is a PWM signal, the description of it and the registers available to control it surely seems "backlight" to me. In particular No, the reason for exposing a backlight is that while while the thing certainly is a PWM signal, the description of it and the registers available to control it surely seems "backlight" to me. > 1. It means a bit of re-inventing the wheel. It's not a very big > wheel, though, I'll give you. ...but it's still something. > The main problem I saw with exposing this as a PWM was the fact that we have both period and frequency to control... > 2. I'm not sure why you'd want to, but in theory one could use this > PWM for some other purposes. It really is just a generic PWM. Your > change prevents that. > ...and in the even that you use it as a "generic" PWM I'd expect that the specified period is related to the frequency of the signal. But the period is documented to be related to the number of brightness steps of the panel. > > > > Drop the #pwm-cells and instead expose a new property to configure the > > granularity of the backlight PWM signal. > > > > Signed-off-by: Bjorn Andersson > > --- > > .../devicetree/bindings/display/bridge/ti,sn65dsi86.yaml | 9 ++++++--- > > 1 file changed, 6 insertions(+), 3 deletions(-) > > > > diff --git a/Documentation/devicetree/bindings/display/bridge/ti,sn65dsi86.yaml b/Documentation/devicetree/bindings/display/bridge/ti,sn65dsi86.yaml > > index f8622bd0f61e..e380218b4646 100644 > > --- a/Documentation/devicetree/bindings/display/bridge/ti,sn65dsi86.yaml > > +++ b/Documentation/devicetree/bindings/display/bridge/ti,sn65dsi86.yaml > > @@ -66,9 +66,12 @@ properties: > > 1-based to match the datasheet. See ../../gpio/gpio.txt for more > > information. > > > > - '#pwm-cells': > > - const: 1 > > - description: See ../../pwm/pwm.yaml for description of the cell formats. > > + ti,backlight-scale: > > + description: > > + The granularity of brightness for the PWM signal provided on GPIO4, if > > + this property is specified. > > + minimum: 0 > > + maximum: 65535 > > A few issues here: > > 1. Maybe call this "num-steps" instead of backlight-scale. That's > essentially what it is, right? Saying how many discrete steps you're > allowing in your backlight? > That would work, I had it as "max-brightness" for a while as well. But I reverted to backlight-scale, because that's the name used in the datasheet. I'm fine with whatever color of the shed though :) > 2. IMO you need the PWM frequency specified, since it can actually > matter. NOTE: once you have the PWM frequency specified, you could > imagine automatically figuring out what "num-steps" was. Really you'd > want it to be the largest possible value you could achieve with your > hardware at the specified frequency. There's no advantage (is there?) > of providing fewer steps to the backlight client. > I guess there's no problem in having a "num-steps" that is unrelated to the number of brightness steps of the panel - but I did distinguish them because the datasheet clearly does so. > 3. Some backlights are specified inverted. It looks like this maps > nicely to the bridge chip, which has a bit for it. Probably nice to > expose this? > Yes, that should be covered. > Of course, if we were just exposing the PWM directly to Linux we could > just use the PWM backlight driver and it'd all magically work. ;-) > Please help me figure out how to properly expose this in the PWM api and I'll be happy to respin it using this - as you say my wheel does look pretty similar... Regards, Bjorn